ZX80 issue 2 kit does not start

Discussions about Sinclair ZX80 and ZX81 Hardware
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crrush
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ZX80 issue 2 kit does not start

Post by crrush » Mon Jul 29, 2019 11:16 pm

Hi, I am pretty new around here so sorry in advance for the noob question.
I have soldered siiue2 replica kit, this one https://www.ebay.co.uk/itm/PCB-ZX80-iss ... SwLgVcjfAU
I also read some early replicas had some problems with MREQ pin on CPU, but this PCB looks okay.

So after soldering done there is literally nothing on the screen, out of sync
I am using a simple composite mod for my telly with a transistor to amplify the video signal, taking it directly from the modulator input.
66D63837-2E10-4C30-96BD-47CEE406077D.png
All chips are getting slightly warm after some time, so I presume they are working, include CPU, but there is no output.
I have no oscilloscope, but simply measuring the voltage on composite gives me 0V. Picking the vid signal gives me something in-between 0.11 and 2.3V
Any tips where I should look would be helpful.
Cheers
66D63837-2E10-4C30-96BD-47CEE406077D.png
ps Don't worry about R12 it has been soldered later on
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1024MAK
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Re: ZX80 issue 2 kit does not start

Post by 1024MAK » Mon Jul 29, 2019 11:46 pm

We need to know more about how your composite transistor circuit is wired up. Is it a NPN transistor, or a PNP. What other components did you use? Maybe you can draw a schematic diagram.

Mark

crrush
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Re: ZX80 issue 2 kit does not start

Post by crrush » Tue Jul 30, 2019 12:09 pm

Hi Mark, thanks for your answer.
I am using the simple mod with NPN mod ripped from here
https://www.bytedelight.com/?page_id=3560
(cheers to Ben, btw)
On my photo below that 2N3904 is flipped, so the wiring is mirrored

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1024MAK
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Re: ZX80 issue 2 kit does not start

Post by 1024MAK » Tue Jul 30, 2019 11:26 pm

Right, except that with the ZX80, you have two different TTL chips providing the sync and video data. The resistors R30 (1kΩ) and R32 (330Ω) then combine these signals, the resulting signal is then fed to the input of the modulator.

But with a 2N3904 NPN transistor wired as an emitter follower configuration, different voltage levels may be needed.

You say that the composite output is 0V. That indicates to me that the NPN transistor is not being turned on, which seems strange to me.

What voltages do you have at either end of R30 and at either end of R32?

Mark

crrush
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Re: ZX80 issue 2 kit does not start

Post by crrush » Thu Aug 01, 2019 3:42 am

I've cheked both R30 and R32, it is 1kOhm and 280 Ohm. But all sides of both resistors (ie all 4 points) give me only 0.16-0.17 V :?

crrush
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Re: ZX80 issue 2 kit does not start

Post by crrush » Thu Aug 01, 2019 11:16 pm

So there is progress, had a closer look I have discovered R24 and R20 are missing (I must have didn't notice that before). Since they are somewhat critical for vid signal, installing them give some interesting consequence.
R32 and R30 give me 3.65 Volts at the output. I think it is okay now. My telly synchs with SYNCH. But the picture is now black with some white random noise, a very weak one. It only occurs if I only use video modulator's input, raw, no amplification.
However, using the amplifier lead only to a black picture as if the amplifier does not work at all. I replaced that transistor but with the same result. I am somewhere quite close but still have some trivial problem, I am sure. :roll:
Here is a video of what I got with modulator input (not the npn-emitter)
https://youtu.be/O5RB_gvwCGc?t=74

Is there a quick way to use somewhat different as an amplifier?

crrush
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Re: ZX80 issue 2 kit does not start

Post by crrush » Sat Aug 03, 2019 9:16 pm

1024MAK wrote:
Tue Jul 30, 2019 11:26 pm

What voltages do you have at either end of R30 and at either end of R32?

Mark
After soldering some vital RR, I came to 3.6V for R30 and 32, same as I do have now after on NPN transistor amplifier.
So I moved to old CRT telly, and I have somewhat out os sync picture
https://www.youtube.com/watch?v=p84BBV2FhsQ

albertusj
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Re: ZX80 issue 2 kit does not start

Post by albertusj » Mon Aug 05, 2019 3:06 pm

I just built one of these:

http://www.myprius.co.za/ZX80.htm

No transistor buffer was necessary, only a back-porch generator. The kit I bought had the WRONG frequency resonator. I had to put in a 6.5536MHz crystal myself.
ZX80/ZX81/ZX Spectrum

crrush
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Re: ZX80 issue 2 kit does not start

Post by crrush » Tue Aug 06, 2019 9:17 pm

albertusj wrote:
Mon Aug 05, 2019 3:06 pm
I just built one of these:

http://www.myprius.co.za/ZX80.htm

No transistor buffer was necessary, only a back-porch generator. The kit I bought had the WRONG frequency resonator. I had to put in a 6.5536MHz crystal myself.
I got 3 pin decriminator as 6.5Mhz supplied with this kit, is that okay?
https://gd3.alicdn.com/imgextra/i1/0/TB ... em_pic.jpg

albertusj
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Re: ZX80 issue 2 kit does not start

Post by albertusj » Wed Aug 07, 2019 4:27 pm

yes, but try a 6.5536MHz crystal also.
ZX80/ZX81/ZX Spectrum

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